Process for the catalytic decomposition of dinitrogen monoxide in a gas stream

ABSTRACT

A process for the catalytic decomposition of dinitrogen monoxide in a gas stream by contacting the gas stream at temperatures of 200°-900° C. and pressures of 0.1 to 20 bar with a catalyst free of noble metals, the catalyst being prepared by combining a spinel CuAl 2  O 3  with another spinel-forming metal component selected from the group consisting of tin, lead, zinc, magnesium, calcium, strontium and barium or mixtures thereof in elemental form or as an oxide or salt, and calcining at temperatures of 300°-1300° C. and under pressures of 0.1-200 bar in order to at least partially liberate the copper from the spinel by replacement with the other metal component.

The present invention relates to a process for the catalyticdecomposition of dinitrogen monoxide which is pure or present in gasmixtures using a catalyst prepared by combining R-Al₂ O₄ where R is anelement of group Ib, VIIb or VIII of the Periodic Table of the Elements,with tin, lead, an element of group IIa or IIb of the Periodic Table ofthe Elements as an oxide or salt or in elemental form, and calcining at300°-1300° C. under 0.1-200 bar.

A review of the energies of activation for the catalytic decompositionof dinitrogen monoxide (laughing gas) on oxide catalysts, especially onmixed oxides, is given in Catalysis Today 4 (1989) 235-251.

The catalysts described therein are unsatisfactory in terms of activity,or useful life, contain costly elements such as noble metals.

It is an object of the present invention to remedy the abovementioneddisadvantages.

We have found that this object is achieved by a novel and improvedprocess for the catalytic decomposition of dinitrogen monoxide which ispure or present in gas mixtures at 200-900° C., which comprisesemploying a catalyst prepared by combining CuAl₂ O₄ with tin, lead, anelement of group IIa or IIb of the Periodic Table of the Elements as anoxide or salt or in elemental form, and calcining at 300°-1300° C. under0.1-200 bar.

The process according to the invention can be carried out by preheatingpure dinitrogen monoxide or gas mixtures containing it or off-gascontaining it in a furnace or heat exchanger to the required reactortemperature of, as a rule, 200°-900° C., preferably 250°-800° C.,particularly preferably 350°-700° C., and then passing it through areaction tube packed with the catalyst described. The preheating of thereaction gas can also take place directly in the reaction tube in apreceding layer of inert material which is at the reaction temperature.The catalyst and/or inert material can be heated by using an externalsource of heat and/or the heat liberated in the decomposition of thedinitrogen monoxide.

It is possible to employ dinitrogen monoxide in extra pure form, ormixed with oxygen or air, or mixed with air containing large amounts ofwater and/or large amounts of other nitrogen oxides such as nitrogenmonoxide and nitrogen dioxide or high concentrations of nitrogen oxidesand other gases such as NO_(x), N₂, O₂, CO, CO₂, H₂ O and inert gases,especially off-gases from adipic acid plants, and it can be selectivelydecomposed to the elements nitrogen and oxygen with negligibledecomposition of other nitrogen oxides to the elements. The content ofnitrogen oxides NO_(x) can, as a rule, be 0-50%, preferably 1-40%,particularly preferably 10-30%, by volume and the N₂ O content can, as arule, be 0.01-65%, preferably 1-55%, particularly preferably 5-45%, byvolume. It is possible, for example, to decompose dinitrogen monoxidemixed with, for example, 20% water and 65% nitrogen dioxide (NO₂)selectively into the elements.

Suitable catalysts are those which can be prepared by combining CuAl₂ O₄with the element as such or oxides or salts of tin, lead, an element ofgroup IIa or IIb of the Periodic Table of the Elements, and, calciningat 300°-1300° C. under 0.1-200 bar. These catalysts contain no noblemetals (Ag, Au, Pd, Pt) and have a BET surface area of 1-350 m² /g.

The starting material may be a solid oxide which is wholly or partly,ie. 1-100%, preferably 10-90%, particularly preferably 20-70%, byweight, a spinel of the composition CuAl₂ O₄ in an Al ₂ O₃ matrix, andthis can be mixed with the same or higher concentration of tin, lead, anelement of group IIa or IIb of the Periodic Table of the Elements, asoxide or salt or in elemental form, and calcined at 300°-1300° C.,preferably 500°-1200° C., particularly preferably 600°-1100° C., under0.1-200 bar, preferably 0.5-10 bar, particularly preferably underatmospheric pressure.

The mixing can take place, for example, by spraying, mechanical mixing,stirring or kneading the ground solid of the composition CuAl₂ O₄,preferably in AL₂ O₃, particularly preferably in Al₂ O₃, or preferablyby impregnating an unground solid of the composition CuAl₂ O₄,preferably in Al₂ O₃, particularly preferably in γ-Al₂ O₃, with tin,lead, an element of group IIa or IIb of the Periodic Table of theElements as oxide or salt (eg. in solution) or in elemental form.

The liberation of the copper in the form of the element or oxide, whichusually leads to fine-particle dispersion, can be induced by partial(>50 mol %, preferably 70 mol %, particularly preferably >90 mol %) orcomplete (100 mol %) replacement of the copper in the spinel in thecalcination step by tin, lead, an element of group IIa or IIb of thePeriodic Table of the Elements in the form of the element, an oxide orsalt-like compound if the resulting spinel is more thermodynamicallystable than the original spinel CuAl₂ O₄. The copper or copper oxidecontent in the catalyst ready for use is 0.1-50%, preferably 1-40%,particularly preferably 5-30%, by weight.

The elements of group IIa or IIb of the Periodic Table of the Elementscan be used in the form of oxides or salt-like compounds or of theelement as such (in metallic form). Examples of salt-like compounds arecarbonates, hydroxides, carboxylates, halides and oxo anions such asnitrites, nitrates, sulfites, sulfates, phosphites, phosphates,pyrophosphates, halites, halates and basic carbonates, preferablycarbonates, hydroxides, carboxylates, nitrites, nitrates, sulfates,phosphates and basic carbonates, particularly preferably carbonates,hydroxides, basic carbonates and nitrates, preferably in the +2oxidation state such as Zn²⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺, especially Zn²⁺and Mg²⁺ or mixtures thereof.

The preparation of the starting oxide of the composition CuAl₂ O₄,preferably in the form of a spinel, is disclosed, for example, in Z.Phys. Chem., 141 (1984), 101-103. It proves advantageous to impregnatean Al₂ O₃ carrier with a soluble compound such as a salt of the cationR, eg. a nitrite, nitrate, sulfite, sulfate, carbonate, hydroxide,carboxylate, halide, halite or halate, and subsequently to decompose theanion to the oxide thermally. Another possibility comprises mixing acompound such as a salt of the cation R with an oxygen-containingaluminum compound, eg. by drying or in suspension, especially byspray-drying, compacting the material, eg. by kneading, whereappropriate by adding a suitable molding aid, molding by extrusion,drying and subsequently calcining to form the spinel. The calcinationcan be carried out at 300°-1300° C., preferably 600°-1000° C.

Doping of aluminum oxide carriers with a large surface area, is. theformation of mixed oxides, increases the thermal stability of thecarrier (eg. DE-A-34 03 328, DE-A-25 00 548, Appl. Catal. 7 (1983)211-220, J. Catal. 127 (1991) 595-604). The foreign ions mayadditionally contribute to the catalytic activity of the catalyst. Thefollowing elements may generally be employed for the doping: alkalimetals, alkaline earth metals, rare earth metals, Sc, Ti, V, Cr, Mn, Fe,Co, Ni, Zn, Y, Zr, B, Si, Ge, Sn, Pb, P, Bi. The degree of replacementof aluminum oxide can be, for example, 0.01-20% by weight.

The size of the copper oxide crystallites in the unused catalyst is1-100 nm, preferably 3-70 nm, particularly preferably 5-50 nm. The sizecan be determined, for example, by XRD (X-ray diffraction) or TEM(transmission electron microscopy).

The catalysts according to the invention contain mesopores of 2-20 nmand macropores of more than 20 nm and have BET surface areas of 1-350 m²/g, preferably 10-200 m² /g, particularly preferably 25-150 m² /g, andporosity of 0.01-0.8 ml/g.

The catalysts which are preferably employed in the process according tothe invention generally contain 0.1-50%, in particular 2-30%, by weightof copper oxide based on the weight of the aluminum oxide. Thespinel-forming metal is present in a concentration which is the same asor higher than that of copper (mol/mol).

The GHSV is, as a rule, 500-50,000 l (STP) gas/1 cat*h, preferably1500-20,000 l (STP) gas/1 cat*h.

EXAMPLES

Decomposition of dinitrogen monoxide

a) The apparatus used for the adiabatic procedure is a Hasteloy Creaction tube which is 800 mm long and is divided into heating andreaction zones. The internal diameter is 18 mm. In order to be able tomeasure the temperature profile in the tube, an inner tube which has anexternal diameter of 3.17 mm and in which a thermoelement can easily bedisplaced was inserted. To improve heat transfer, the reactor was packedwith inert material (steatite) in the heating zone.

b) Alternatively, however, the reaction can also be carried out underquasi-isothermal conditions in a salt bath reactor. The heat transferagent is a melt composed of 53% by weight KNO₃, 40% by weight NaNO₂ and7% by weight NaNO₃. The decomposition is carried out in a Hasteloy Creaction tube which is 600 mm long. The internal diameter is 14 mm. Thegas is heated to the reaction temperature in a longer preheating zone.In order to be able to measure the temperature profile in the tube, onceagain an inner tube which has an external diameter of 3.17 mm and inwhich a thermoelement can easily be displaced was inserted.

In each case 40 ml of catalyst (1.5-2 mm chips) were tested.

The decomposition of N₂ O in a gas mixture typical of the off-gas froman adipic acid plant was tested.

Typical gas composition:

N₂ O: 23% by volume

NO₂ : 17% by volume

N₂ : 47% by volume

O₂ : 7.5% by volume

H₂ O: 3% by volume

CO₂ : 2.5% by volume

GHSV: 4,000 l (STP) gas/l cat*h

Preparation of the catalysts

EXAMPLE 1

A mixture of 284 g of Puralox® SCF (from Condea), 166 g of Pural® SB(from Condea) and 100 g of CuO (from Merck) was headed with 20 ml offormic acid (dissolved in 140 ml of H₂ O) for 0.75 h, extruded to 3 mmextrudates, dried and calcined at 800° C. for 4 h.

71.4 g of the CuAl₂ O₄ -containing aluminum oxide carrier (water uptake:69.1%) were impregnated twice with 49 ml of an aqueous solution whichcontained nitric acid (pH 3) and 32.6 g of Zn(NO₃)₂ and then left atroom temperature for one hour. The impregnated carrier was dried toconstant weight at 120° C. and finally calcined at 600° C. for 4 h.

EXAMPLE 2

A mixture of 346 g of Puralox® SCF (from Condea), 180 g of Pural® SB(from Condea) and 120 g of CuO (from Merck) was headed with 18 ml offormic acid (dissolved in 390 ml of H₂ O) for I h, extruded to 3 mmextrudates, dried and calcined at 800° C. for 4 h.

85.2 g of the CuAl₂ O₄ -containing aluminum oxide carrier (water uptake:70%) were impregnated three times with 47 ml of an aqueous solutionwhich contained nitric acid (pH 2.5) and 45.2 g of Mg(NO₃)₂.6 H₂ O andthen left at room temperature for one hour. The impregnated carrier wasdried to constant weight at 120° C. and finally calcined at 700° C. for4 h.

EXAMPLE 3

A mixture of 288.4 g of Puralox® SCF (from Condea), 350 g of Pural® SB(from Condea) and 140 g of CuO (from Merck) was headed with 25 ml offormic acid (dissolved in 530 ml of H₂ O) for 1 h, extruded to 3 mmextrudates, dried and calcined at 800° C. for 4 h.

65.9 g of the CuAl₂ O₄ -containing aluminum oxide carrier (water uptake:60.3%) were impregnated twice with 47 ml of an aqueous solution whichcontained nitric acid (pH 3.1) and 34.7 g of Ca(NO₃)₂ and then left atroom temperature for one hour. The impregnated carrier was dried toconstant weight at 120° C. and finally calcined at 700° C for 4 h.

COMPARATIVE EXAMPLE 1

A catalyst was prepared as described in DE-A-40 29 061. 150 g ofcommercial aluminum oxide carrier (BET surface area 1.7 m² /g; wateruptake 29.2% by weight) was impregnated with 100 ml of aqueous solutionwhich contained 41.7 g of AgNO₃ and then left to stand at roomtemperature for one hour. The impregnated carrier was dried to constantweight at 120° C. and finally calcined at 700° C. for 4 h. The catalystobtained in this way contained 14.6% by weight of silver and had a BETsurface area of 1.12 m² /g.

COMPARATIVE EXAMPLE 2

The palladium catalyst on alpha-aluminum oxide preferred in DE-A-35 43640 was prepared. 200 g of galpha-aluminum oxide (BET surface area 20.2m² /g) were impregnated with NaOH and dried at 120° C. This carrier wasimpregnated with 96 ml of an aqueous sodium tetrachloropalladate(II)solution containing 1.29 g of Pd and then left to stand at roomtemperature for three hours. The Pd²⁺ -containing carrier was treatedwith hydrazine to reduce the Pd²⁺. The catalyst was subsequently washeduntil free of chlorine and dried to constant weight at 120° C. Thecatalyst obtained in this way contained 0.64% by weight of palladium.

COMPARATIVE EXAMPLE 3

A catalyst was prepared as described in DE-A-41 28 629. 225 g of Pural®SB were kneaded with 25 g of La(NO₃)₃ and 12.5 g of formic acid for 3 h,extruded, dried and calcined. 64.10 g of this (BET surface area 183 m²/g; water uptake 76% by weight) were impregnated with 50.9 ml of anaqueous solution which contained 17.8 g of AgNO₃ and then left to standat room temperature for one hour. The impregnated carrier was dried toconstant weight at 120° C. and finally calcined at 700° C. for 4 h. Thecatalyst obtained in this way contained 14.5% by weight of silver andhad a BET surface area of 156 m² /g.

Test results

a) Adiabatic process

    ______________________________________                                                 Running time Temperature                                                                              Conversion                                   Catalyst (h)          (°C.)                                                                             (%)                                          ______________________________________                                        1        1036         480        >99.9                                        2        1025         485        >99.9                                        3        1013         485        >99.9                                        C1        150         610        97.5                                         C2        112         640        66.5                                         C3        280         530        >99.9                                        ______________________________________                                    

The test results (catalysts 1 to 3) make it clear that the newlydeveloped silver-free catalysts are both more active and more stable inan adiabatic process than are prior art catalysts C1 to C3.

    ______________________________________                                                Running time                                                                             Salt bath    N.sub.2 O conversion                          Catalyst                                                                              (h)        temperature (°C.)                                                                   (%)                                           ______________________________________                                        1       48         540          98.0                                          2       48         540          97.2                                          3       48         540          97.4                                          C3      48         540          41.0                                          ______________________________________                                    

The test results (catalysts 1 to 3) show that differences in activityare much more clearly evident in an isothermal process than in anadiabatic process where the energy released by the decomposition of N₂ Omakes a large contribution to the decomposition. The superiority of thenewly developed silver-free catalysts compared with prior art catalystsC1 to C3 can be clearly demonstrated by carrying out an isothermalreaction.

We claim:
 1. A process for the catalytic decomposition of dinitrogenmonoxide in a gas stream which comprises contacting the gas stream attemperatures of 200°-900° C. and pressures of 0.1 to 20 bar with acatalyst free of noble metals, said catalyst being prepared by combininga spinel CuAl₂ O₄ in an Al₂ O₃ matrix with an additional spinel-formingmetal component selected from the group consisting of tin, lead, zinc,magnesium, calcium, strontium and barium or mixtures thereof inelemental form or as an oxide or salt, and calcining at temperatures of300°-1300° C. and under pressures of 0 1-200 bar for partial or completeliberation of the copper from said spinel in the form of its oxide byreplacement with said additional metal component.
 2. A process asclaimed in claim 1, wherein said metal component is present as the oxidein the 2+ oxidation state when replacing the copper oxide of the spinel.3. A process as claimed in claim 1, wherein said metal component isselected from the group consisting of zinc, magnesium, calcium,strontium and barium or mixtures thereof.
 4. A process as claimed inclaim 3, wherein said metal component is zinc magnesium or mixturesthereof.
 5. A process as claimed in claim 3, wherein the metal componentis zinc.
 6. A process as claimed in claim 1, wherein the catalyst has aBET surface area of 1-350 m² /g.
 7. A process as claimed in claim 1,wherein the catalyst has a CuO content of 0.1-50% by weight.
 8. Aprocess as claimed in claim 1, wherein the porosity of the catalyst is0.01-0.8 ml/g.
 9. A process as claimed in claim 1, wherein the gasstream contacted with said catalyst contains up to 50% by volume ofother nitrogen oxides.
 10. A process as claimed in claim 1, wherein thegas stream contacted with said catalyst contains from 0.01-65% by volumeof dinitrogen monoxide.
 11. A process as claimed in claim 1, wherein thecatalyst contains 1-40% by weight of copper oxide, based on the weightof the aluminum oxide, and the additional metal is present in a molarconcentration which is the same or higher than that of copper.
 12. Aprocess as claimed in claim 11, wherein the catalyst contains mesoporesof 2-20 nm and macropores of more than 20 nm and has a BET surface areaof from 10-200 m² /g and a porosity of 0.01-0.8 ml/g.